The present invention relates to a protection circuit used in an electronic ballast to reduce through-lamp leakage currents. More particularly, the present invention relates to a safety control circuit used in a series resonant ballast driving multiple lamps to reduce the magnitude of through-lamp leakage currents whenever one or more lamps are removed from at least one of the lamp terminals of the lamp fixture.
Those skilled in the design and operation of electronic ballasts have long recognized the problems created by the flow of high magnitude through-lamp leakage currents when lamps driven by such ballasts are removed from the lamp fixture by a service person. A shock hazard situation develops when such a service person is in contact with the earth ground while holding onto one end of a lamp while the other end is still in the lamp terminal. In such a case, a high magnitude of current can flow to ground through the lamp and the person, causing the holder to suffer an electric shock.
Several solutions for providing a safe lamp ballast system for addressing problems similar to the one stated above have been proposed or used in the prior art, with limited success. For example, in U.S. Pat. No. 4,983,887, issued to Nilssen, a circuit is designed to limit the magnitude of high frequency through-lamp leakage current distributed to an open lamp outlet. This is done by providing a negative feedback control circuit which reduces the forward conduction duty cycle of both of the transistors of the inverter of the ballast whenever the peak through-lamp leakage current magnitude exceeds a predetermined level. While this circuit does reduce the magnitude of the high frequency through-lamp leakage current, it requires that the duty cycle of both the transistors be reduced at the same time and further requires a foreknowledge of the magnitude of current that is allowable. This means that additional sensing and control circuits are required which makes the total circuit more complex and drives up the cost of the ballast.
Others have attempted to reduce the magnitude of these through-lamp leakage currents through shut-down circuits. Shut-down circuits suspend operation of the inverter when a lamp is removed from the lamp fixture that is electrically connected to the ballast. For example, U.S. Pat. No. 4,461,980, issued to Nilssen, describes a protection circuit that disables the ballast inverter approximately one second after a fluorescent lamp is removed from at least one of the lamp terminals of the lamp fixture. This method uses a clamping current to generate heat and actuate a bimetallic switch which causes a short-circuit in the feedback loop of the control circuit, forcing the inverter oscillations to stop. However, this method only addresses a single lamp instant start ballast and requires precise adjustment of the response time of the feedback loop to function properly. Moreover, it requires additional circuitry to reinitiate inverter oscillations, once every 30 seconds after the inverter has been disabled. Shut-down circuits in rapid-start ballasts driving one or two lamps have also been proposed and described. These require additional delay circuits to get past the pre-heat stage during lamp starting. Also, if the delay is not accurate, false triggering can occur, resulting in premature shut-down of the lamps.
Still others have attempted to solve the problem of dangerous through-lamp leakage currents by warning persons of the situation through pulsing circuits installed in the ballasts. These circuits control the inverter transistors of the ballast and force the lamps to flash whenever one of the lamps is removed from a lamp terminal of the lamp fixture. The control circuits that have been used to achieve this are quite sophisticated, however, and add significantly to the cost of producing such a design.
What is needed then, and not found in prior art, is an efficient, simple and inexpensive way of detecting and reducing hazardous through-lamp leakage currents to safe levels whenever one or more lamps are removed from their lamp terminals of the lamp fixture, especially in the case of a ballast driving multiple lamps.